Benchtop flow cytometers are the current standard for high speed, multi-parametric cell sorting. However, these droplet-based sorting systems are often open and have several intrinsic drawbacks. Contamination from the environment, operators, and samples are major issues for downstream cell culture and analysis. Cell loss during transfer is also hard to prevent. In addition, infectious samples need to be treated with extreme caution because droplet-based sorting mechanisms generate biohazardous aerosols.
Microfluidic approaches have the potential to solve these issues by providing a sorting system on a disposable chip to enclose all cells and liquids in a closed, safe, and sterile environment. This eliminates the need for vacuum containment systems and facility additions to protect operators against aerosolized pathogen exposure in conventional droplet-based fluorescence activated cell sorting (FACS). Furthermore, a microfluidic approach is advantageous for handling small numbers of cells (100-100,000) with high yield, which is difficult to achieve with conventional FACS. This feature is particularly beneficial in applications involving precious cells, such as primary cells that cannot be expanded to large populations. In addition, other lab-on-a-chip devices, the potential exists for further functionality integrated on-chip, such as sample preparation, cell incubation, chemical analysis, PCR, or other assays of the sorted populations. Specifically, these capabilities would enable sorting and molecular analysis of rare cells in blood such as circulating tumor cells (CTCs), providing powerful molecular diagnostic information concerning cancer drug resistance in a non-invasive manner for personalized therapies.
Although a variety of different physical on-chip switch mechanisms have been proposed, none has simultaneously satisfied the requirements of high throughput, purity, and recovery of live, unstressed mammalian cells (see, e.g., Applegate et al. (2006) Lab on a Chip, 6: 422-426; Fu et al. (2002) Analytical Chem., 74: 2451-2457; Fu et al. (1999) Nature Biotechnology, 17: 1109-1111; Ho et al. (2005) Lab on a Chip, 5: 1248-1258; Holmes et al. (2007) Lab on a Chip 7: 1048-1056; Idota et al. (2005) Advanced Materials, 17: 2723-; Kim et al. (2007) Rev. Sci. Inst., 78(7): 074301.; Shirasaki et al. (2006) Analyt. Chem., 78: 695-701; Wang et al. (2005) Nature Biotechnol., 2383-8237). Some of the first demonstrations of the sorting of bacterial cells relied on electrokinetic mobilization of fluid through a microfluidic network, achieving rates of 1-20 cells/sec. Unfortunately, this method is limited by the difficulty of maintaining cell viability under high electric fields, particularly for eukaryotic cells, and by buffer incompatibilities. Hydrodynamic flow control based on either on-chip or off-chip fluidic valves has been demonstrated for sorting living cells; in this case, preserving cell viability is less of a problem. However, because of the slow pneumatic mechanical switch and the relatively large volume of fluid displaced in every switch cycle, cell sorting demonstrations using hydrodynamic switching are slow. The recently demonstrated PZT method (Cho et al. (2010) Lab on a Chip, 10: 1567-1573) is able to achieve 1000 cells/sec. However, this sorting speed is still 1˜2 orders of magnitude slower than commercial FACS.